This invention relates to a color cathode ray tube device with an in-line electron beam arrangement.
The envelope of a color cathode ray tube device consists of: a neck in which are installed three electron guns that generate three electron beams and are aligned in the horizontal direction; a face plate having a phosphor screen; and a funnel disposed between the neck and the face plate.
The three electron beams generated from the in-line type electron guns, mounted in a horizontally in-line arrangement, are directed onto the phosphor screen, which is formed coated with phosphor layers, causing the phosphor layers to emit light. In order to achieve good color reproduction with the light emitted from the phosphor layers, the electron beams must be made to impinge selectively on prescribed phosphor layers. This is achieved by arranging a shadow mask formed with a large number of apertures close to the face plate.
The in-line electron guns incorporate separate cathodes and are designed so as to generate three electron beams in a common horizontal plane and bring them to convergence in the vicinity of the face plate. Known methods of bringing the three electron beams to convergence include for example the technique disclosed in U.S. Pat. No. 2,957,106 (Moodey), in which the side beams in the electron beams emitted from the cathodes are bent from the start, and the technique disclosed in U.S. Pat. No. 3,772,554 (Hughes), in which the electron beams are converged by, of the apertures provided in the electron beam electrodes for passage of the three electron beams, displacing those apertures which are on both sides of part electrode slightly to the outside from the centre axes of the electron guns, thereby bending the electron beam by creating a potential gradient in the electric field generated at the displaced portions. Both these methods are widely used.
To make the phosphor screen of a color cathode ray tube display a TV picture, the electron beams must be scanned over the entire surface of the phosphor screen. This is done by mounting a deflection device outside the cone portion of the funnel. Essentially the deflection device comprises horizontal deflection coils for generating a horizontal deflection magnetic field that deflects the electron beam in the horizontal direction and vertical deflection coils for generating a vertical deflection magnetic field that deflects the electron beam in the vertical direction. In practical color cathode ray tubes when the electron beams are deflected by a uniform magnetic field, because of the leakage field that extends beyond the end surface of coils, convergence of the three electron beam spots on the face plate is lost. Various countermeasures have to be adopted to deal with this, so that the spots always converge over the whole surface of the screen. Such a system is termed a "convergence free system". In this system, convergence of the three electron beams over the entire phosphor screen is achieved by making the horizontal deflection magnetic field of pin-cushion form, and making the vertical deflection magnetic field of barrel form. If the vertical magnetic field is uniform, there is overconvergence which increases in degree from the center of the screen towards the top and bottom ends, but with a barreltype magnetic field, convergence can be achieved over the entire screen. As a result, with such a system, a parabolic current generating circuit for convergence compensation and a convergence yoke for generating a convergence compensating magnetic field can be dispensed with, conferring many advantages such as cost saving and productivity gain.
As explained above, the quality of color cathode ray tubes has been improved by many technical developments. However, as large tubes have become common, fresh problems have come to the fore.
One of these problems concerns the shape of the beam spot where the electron beams are brought to convergence on the face plate after being emitted from the electron guns. As shown in FIG. 4(a), in the middle of the screen, where the beams are not subjected to any deflection, the spot S.sub.4a consists simply of a round core Sc, i.e. a region of high electron density. However, as shown in FIG. 4(b), due to non-uniformity of the deflection magnetic field, in the peripheral regions of the screen, where the spot S.sub.4b is subject to deflection, the spot presents a flattened core S.sub.c with vertically extending flares S.sub.f (i.e. portions of lower electron density). As a result, the electron beam size increases at the edges of the screen, producing a deterioration in focussing property and resolution.
Specifically, if we take the horizontal dimension of the core for the case of a 20 inch 90 degree deflection tube as C.sub.H and its vertical dimension as C.sub.V, in the middle of the screen C.sub.H =C.sub.V =1.0mm, but at the extreme end region of the horizontal deflection the core has a very flattened shape with C.sub.H =20mm and C.sub.V =0.3mm. Also, the dimension F.sub.V from the top to the bottom of the flares is 1.5mm. These values are for the case where the electron beam is deflected in the horizontal direction only. In the corners of the screen, where a vertical deflection is added to the horizontal deflection, the dimensions are even more distorted.